Method for generating high-contrast images of semiconductor sites via one-photon optical beam-induced current imaging and confocal reflectance microscopy

ABSTRACT

A method is disclosed that permits the generation of exclusive high-contrast images of semiconductor sites in an integrated circuit sample ( 19 ). It utilizes the one-photon optical beam-induced current (1P-OBIC) image and confocal reflectance image of the sample that are generated simultaneously from one and the same excitation (probe) light beam that is focused on the sample ( 19 ). A 1P-OBIC image is a two-dimensional map of the currents induced by the beam as it is scanned across the circuit surface. 1P-OBIC is produced by an illuminated semiconductor material if the excitation photon energy exceeds the bandgap. The 1P-OBIC image has no vertical resolution because 1P-OBIC is linear with the excitation beam intensity. The exclusive high-contrast image of semiconductor sites is generated by the product of the 1P-OBIC image and the confocal image. High-contrast image of the metal sites are also obtained by the product of the complementary OBIC image and the same confocal image.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage entry under 35 U.S.C.371, of International Application No. PCT/PH2002/000013, filed Jul. 9,2002.

FIELD OF THE INVENTION

The invention relates to a method of precisely determining the locationof defects in an integrated circuit.

BACKGROUND OF THE INVENTION

Optical beam induced current (OBIC) imaging is widely employed forfailure or defect detection of pn junctions, inter-level shorts,transistor states, etc, in integrated circuits (IC). An OBIC image is amap of the current magnitudes that are induced when a (focused) opticalbeam is scanned across an IC sample. Scanning confocal microscopy withits focused probe beam, is readily combined with OBIC imaging to producea pair of confocal reflectance- and OBIC images of the sample from oneand the same beam scan.

The one-photon absorption OBIC (1P-OBIC) is produced by an illuminatedsemiconductor material if the probe photon energy exceeds thesemiconductor bandgap E_(b) i.e. λ_(p)≦hc/E_(b), where λ_(p) issingle-photon wavelength, h is the Planck's constant and c is the speedof light in vacuum. 1P-OBIC is proportional to the probe beam intensityand the measured 1P-OBIC signal is an integrated effect along theoptical beam path. Unlike confocal images which are high-contrastdisplays of the reflectance of a three-dimensional sample, thecorresponding 1P-OBIC image of the same sample has low contrast andlacks vertical resolution.

Two-photon OBIC (2P-OBIC) has been demonstrated to generatehigh-contrast images of semiconductor sites in an IC. 2P-OBIC utilizesan excitation beam with a wavelength λ_(2P)>hc/E_(b). 2P-OBIC isproportional to the square of the beam intensity and is highly localizedwithin the focal volume of the excitation beam. Another technique forobtaining high-contrast 1P-OBIC images is via near-field microscopy witha subwavelength fiber Up. A major drawback of 2P-OBIC is the high costof a femtosecond laser source. Image generation in near-field microscopyis slow and unsuitable for generating large image fields. It is alsosensitive to ambient experimental conditions.

Here, we present a procedure for generating high-contrast images ofsemiconductor sites in the IC from their 1P-OBIC image and confocalreflectance image which are both obtained from the same focused beam.The procedure utilizes the following properties: (1) only semiconductormaterials produce an OBIC signal, and (2) confocal reflectance imagesare optically-sectioned images of both metallic and semiconductorsurfaces. We show that the product of the low-contrast 1P-OBIC image andthe confocal image results in a high-contrast (axial-dependent) map thatreveals only the semiconductor sites in the confocal image. Similarly,the product of the complementary to the 1P-OBIC image and the confocalimage yields an optically sectioned image exclusively of thenon-semiconductor sites in the IC sample.

Another advantage of 2P-OBIC imaging over 1P-OBIC is realized whenobserving in the presence of an intervening highly scattering mediumbetween the focusing lens and the semiconductor material. Because thescattered intensity is inversely proportional to a power of the incidentwavelength and that λ_(2P)=2λ_(p), a much greater percentage of the 2Pexcitation photons is delivered at the focal volume of a 2P excitationbeam than their 1P counterparts for the same scattering medium andnumerical aperture (NA) of the focusing lens. The scatter-inducedbroadening of the axial distribution of the 2P-OBIC signal is lesssevere than that of 1P-OBIC. In 1P fluorescence excitation microscopywith large-area photodetector, the effect of scattering is to degradethe signal-to-noise ratio of the generated images.

It is worth noting that confocal microscopy is also robust against theunwanted effects of scattering by an intervening medium. Thephotodetector pinhole acts a spatial filter that permits only thedetection of photons emanating from the focal volume of the probe beam.The undesirable image contribution of the photons from the out-of-focusplanes can be minimized through careful choice of the pinhole size.

1P excitation (1PE) confocal microscopy can be done with objectives ofrelatively low NA values but long working distances—an advantage that isof practical importance for wide-field observation and when dealing withthick samples. In contrast, 2PE imaging requires objectives with largeNA values to generate sufficiently high intensities at the focal spotbecause the 2PE absorption cross-section is much smaller than its 1PEcounterpart. Such objectives however, normally have short workingdistances that limit our ability to scan axially thick samples at longdepths. Aberration-free high NA objectives with long working distancesare quite expensive to manufacture.

SUMMARY OF THE INVENTION

The present invention, in one broad sense, is about the discovery thatexclusive high-contrast images of semiconductor sites can be generatedquickly and accurately from the 1P-OBIC image and the confocalreflectance image which are obtained via one and the same excitationbeam that is focused on the IC sample.

The process makes use of the fact that: (1) confocal images areoptically-sectioned images while 1P-OBIC images are exclusivelow-contrast images of semiconductor sites, and (2) both the confocalimage and 1P-OBIC image are produced with an optical beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description of the invention can be readily appreciated inconjunction with the accompanying drawings, in which

FIG. 1 is an optical set-up of beam-scanning optical microscope forsimultaneous confocal reflectance and 1P-OBIC imaging. The opticalexcitation power is controlled via a neutral density filter (10).

FIG. 2 presents a comparison of confocal (a) and 1P-OBIC (b) images atvarious axial locations (Δz=1 micron, 128 by 128 pixels, and image size:30 micron×30 micron). The images are (raw) outputs of the optical set-updescribed in FIG. 1.

FIG. 3 shows the exclusive images of semiconductor sites (a) and metalsites (b) at various axial locations (Δz=1 micron, 128 by 128 pixels,and image size: 30 micron×30 micron). The images are derived using theconfocal and 1P-OBIC images in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The teachings of the present invention can be readily understood withreference to the accompanying figures, in which details of the preferredmanner of practicing the present art are described. Accordingly, personsof skill in the appropriate arts may modify the disclosures of thepresent invention but still obtain the favorable results describedherein. Since the understanding of the underlying principles aboutoptical image formation are key to the process, a description of thesame is in order.

Underlying Principle

The amplitude point spread function (PSF) of a confocal microscope (witha point light source and a point detector) is given by: h₁(x, y, z)h₂(x,y, z)=h₁h₂, where h₁ and h₂ are the point spread function of thefocusing and collector lens, respectively. In a confocal reflectancemicroscope, h₁=h₂=h. The confocal intensity image i_(r)(x, y, z) that isgenerated from a three-dimensional object with a reflection amplitudedistribution o_(r)(x, y, z) is described by:i _(r)(x, y, z)=|o _(r)(x, y, z){circle around (×)}h ²(x, y, z)|²   (1)where {circle around (×)} represents a convolution operation. Metals andsemiconductors surfaces have relatively high |o_(r)(x, y, z)|² values.The intensity PSF which is the confocal image that is produced by apoint object is: |h²(x, y, z)|². The optical sectioning capability of aconfocal microscope that permits the generation of high-contrast imagesis a consequence of the |h²(x, y, z)|²-behavior of the PSF.

The same focused beam of the confocal microscope generates the 1P-OBICsignal whose strength is proportional to the beam intensity and dependsnot only on the 1P absorption cross-section and the incident beam powerbut also on the optical path length. This implies that the measured1P-OBIC signal i_(s) does not exhibit a z-dependence and is calculatedas:

$\begin{matrix}{{i_{s}( {x,y} )} = {\int_{- \infty}^{\infty}{{{o_{s}( {x,y,z,} )} \otimes {{h( {x,y,z} )}}^{2}}\ {\mathbb{d}z}}}} & (2)\end{matrix}$where o_(s)(x, y, z) represents the distribution of the semiconductormaterial in the sample and i_(s)(x, y)≧0. For non-semiconductormaterials (e.g. metals, dielectrics), o_(s)(x, y, z)=0. Because i_(s)(x,y) has no axial dependence, a 1P-OBIC image has low contrast andcontains no information about the depth distribution of thesemiconductor sites in the sample. It has already been reported earlierthat the 1P-OBIC image lacks vertical resolution.

However, an exclusive high-contrast image of semiconductor sites can bederived from i_(r)(x, y, z) and i_(s)(x, y) by taking their imageproduct: s(x, y, z)=i_(r)(x, y, z)i_(s)(x, y), where s(x, y, z)≧0. Fromthe properties of o_(r)(x, y, z) and o_(s)(x, y, z), it is evident thats(x, y, z) is non-zero only for semiconductor materials. From Eqs (1)and (2), the associated PSF for the product image is given by: h⁴(x, y,z)∫h²(x, y, z)dz=h⁶(x, y)h⁴(z), where we have assumed that: h(x, y,z)=h(x, y)h(z), in the final expression. Therefore, s(x, y, z) providesan exclusive map of the semiconductor sites and exhibits the verticalresolution of i_(r)(x, y, z).

An exclusive high-contrast image of the metallic sites is obtained fromthe product: m(x, y, z)=i_(r)(x, y, z)i_(m)(x, y), where: i_(m)(x,y)=κ−i_(s)(x, y), and κ is a constant that represents the highest s(x,y, z) value that is possible for a given optical set-up. In practice,the sample is scanned by the focused beam at a sampling interval thattakes into account the central spot size of h(x, y, z) and the Rayleighresolution criterion. The scanned confocal and 1P-OBIC images arerepresented by {i_(r)(i, j, k)} and {i_(s)(i, j, k)} respectively,where: x=iΔx, y=jΔy, and z=kΔz, i, j=1, 2, . . . J; and k=1, 2, . . . ,K. The sampling intervals are given by Δx, Δy, and Δz, respectively. Inour experiments, an 8-bit analog-to-digital converters are utilized forboth i_(r)(x, y, z) and i_(s)(x, y) so that: 0≦i_(r)(i, j, k)≦255; and0≦i_(s)(i, j, k)≦κ=255.

The algorithm for generating each element in scanned product image {s(i,j, k)}={i_(r)(i, j, k)i_(s)(i, j, k)}, has a computational complexity oforder 1. It could be implemented very quickly. The s(i, j, k)-values arealso not susceptible to rounding-off errors which are attendant initerative reconstruction algorithms with high computational complexity.

Experimental Set-up

A beam-scanning reflectance microscope was constructed for both 1 P-OBICand confocal imaging (FIG. 1). Via a beam splitter (11), the output beamof laser is directed to a scanning mirror system that is composed of twogalvanometer mirrors (General Scanning Model G115) for x (12) and y (13)scanning, and two lenses (L1, L2) (14, 15) that constitute a 4 ftransfer lens. Another pair of lenses (16, 17) expands and collimatesthe scanned beam and inputs it to an optical microscope assembly. Aninfinity-corrected objective lens (18) focuses the beam into the exposedtop surface of the integrated circuit sample (19). The beam is directedusing a plane mirror (20). Precise 2D scan control of the focused beamis achieved via a pair of digital-to-analog converters (21).

The reflected light is collected back by the same objective lens (18)and focused by lens (22) towards a pinhole that is placed in front ofphotodetector (23). The 1P-OBIC is measured by inputting the output ofthe pin that is nearest to the probe surface area to acurrent-to-voltage converter composed of an operational amplifier and afeedback resistor (24). The other converter input is the commonreference (25) for the electronic circuits including the IC sample. A1P-OBIC signal is induced because the bandgap E_(b) is less than theexcitation photon energy.

The control of the instrument, the data acquisition system and thepost-detection processing are implemented via a personal computer (26).Both the 1P-OBIC signal and the photodetector signal are sampled to thecomputer by a pair of analog-to-digital converters (27).

CONCLUSION

An efficient and economical method has been disclosed that permits thegeneration of exclusive high-contrast images of semiconductor sites inan integrated circuit sample. It utilized the one-photon opticalbeam-induced current (1P-OBIC) image and confocal reflectance image ofthe sample that are generated simultaneously from one and the sameexcitation (probe) light beam that is focused on the sample. Theexclusive high-contrast image of semiconductor sites is generated by theproduct of the 1P-OBIC image and the confocal image. High-contrast imageof the metal sites are also obtained by the product of the complementaryOBIC image and the same confocal image.

That which is claimed is:
 1. A method of high contrast imaging ofsemiconductor and metallic sites in an integrated circuit (IC) thatsimultaneously produces two separate exclusive high-contrast images ofsaid IC from one light source, the method comprising: exciting said ICwith a focused excitation beam from a light source; transversely andaxially scanning said IC by said focused excitation beam; producingsimultaneously a high-contrast confocal reflectance image i_(r)(x, y, z)and a low contrast one-photon optical beam-induced current image(1P-OBIC) i_(s)(x, y) of said IC; deriving a first exclusivehigh-contrast image s(x, y, z) of said semiconductor sites of said ICfrom a pixel to pixel product of said 1P-OBIC image and said confocalreflectance image using the equation: s(x, y, z)=i_(r)(x, y, z)i_(s)(x,y) where s(x, y, z)>0; and deriving a second exclusive high-contrastimage m(x, y, z) of said metallic sites of said IC from a product of acomplementary to said 1P-OBIC image and said confocal reflectance imageusing the equation: m(x, y, z)=i_(r)(x, y, z)i_(m)(x, y) where i_(m)(x,y)=κ−i_(s)(x, y) and κ is a constant that represents the highest s(x, y,z) value that is possible for a given optical set-up.
 2. The method ofclaim 1, wherein said focused excitation beam is a beam-scanningconfocal reflectance microscope.
 3. The method of claim 1, wherein saidlight source is selected from the group consisting of a laser and aspectrally filtered light source with a broadband spectrum.
 4. Themethod of claim 3, wherein said device includes a scanning mirror systemhaving two galvanometer mirrors for x and y scanning, and two lensesthat constitute a 4 f transfer lens, wherein said light source isdirected to said scanning mirror system.
 5. The method of claim 4,wherein said device includes another pair of lenses that expand andcollimate said excitation beam and inputs said excitation beam to anoptical microscope assembly.
 6. The method of claim 5, wherein saiddevice includes an Infinity-corrected objective lens that focuses saidexcitation beam into said IC.
 7. The method of claim 6, wherein saiddevice includes a pair of digital-to-analog converters to achieveprecise two-dimensional scan control of said focused excitation beam. 8.The method of claim 7, wherein said device provides reflected light thatis collected back by said Infinity-corrected objective lens and focusedby a lens towards a pinhole that is placed in front of a photodetector.9. The method of claim 8, wherein said 1P-OBIC is measured by inputtingan output of said pinhole that is nearest to a probe surface area to acurrent-to-voltage converter composed of an operational amplifier and afeedback resistor.
 10. The method of claim 9, wherein said deviceincludes another converter input that is a common reference forelectronic circuits including said IC.